This invention relates to method and apparatus for preparing the longitudinal edge of a steel plate in a UOE process, and more particularly, to an improved method and apparatus which effectively broadens the range of widths and thicknesses of steel plates utilizable in the manufacture of pipes.
In making steel pipes using the UOE method, a steel plate is first cut and formed into bevels at both longitudinal edges followed by U-ing and O-ing and then the bevels are abutted together and subsequently, subjected to welding to create a seam along the abutted beveled edges. If the pressing load is insufficient during the O-ing, the butted edges would not be sufficiently effected with the press forming and the edge parts would be abutted in a straight condition to produce so-called peaking, which is a disorder out of a true circle.
For solving such problems, the conventional method involved bending a steel plate at its longitudinal edges by a predetermined curvature prior to U-ing. One of the known processes uses an upper roll and a lower roll to bend the edges of a steel plate. This process is suitable for bending the edges of a thin steel material. But disadvantageously, if the steel is thick or the size of its width is small, this prior method and apparatus require that the forming force be rapidly increased, and consequently, that the rolls be large scaled since they should be kept rigid. In another prior method and apparatus, an upper die and a lower die (which is called a C-press) are employed using oil pressure for strongly pressing the edges of a steel plate. Disadvantageously, in this method and apparatus, when a thick steel plate is acted upon, an outer edge part of the lower die slips from contact by plastic deformation due to reaction force of theplate, so that a clearance is created between the upper surface of the plate toward the edge and the outer portion of the upper die, and hence the steel plate is not acted upon with any degree of force at the edge. Thus, the steel plate is only acted upon and bent at the parts which contact the two dies. The bending moment does not effectively act upon the entire surface area and after the bending process, straight parts remain at the ends of the side edges, resulting in the so-called peaking phenomenon.
In order to solve the foregoing problem, a bending system was proposed in Japanese Patent Application SN 53-135,877 comprising a male die and a female die between which a side edge of a steel plate may be disposed. The female die is supported turnably around a fulcrum, which fulcrum is movable in a vertical direction to provide pressing force onto the steel plate. The female die can concentrate load to the end portion of the side edges of the steel plate and desired bending can be attained. However, disadvantageously, the amount of force required to move upwardly the female die is substantial in that the force is applied directly vertically to the fulcrum.
Thus, there is an urgent need in the prior art for a method and apparatus which can effectively prepare the longitudinal edges of a steel plate for the UOE process of pipe making, while utilizing practical amounts of pressure and which can be utilized for greater thicknesses of steel plate and smaller widths of the edges.